In sewing machines, a needle thread is inserted in a needle while being guided by a thread take-up lever, and a bobbin thread is held in a hook. The needle, the hook, and the thread take-up lever are linked one another by an upper shaft that drives a needle bar, a lower shaft that drives the hook, and a toothed belt engaged with the upper and lower shafts. That is, when the upper shaft is driven by drive power, etc., from a motor, the lower shaft also rotates, and the needle and the hook and the thread take-up lever are actuated relative to one another. Sewing machines catch, through the tip of the hook, a thread loop formed by the needle thread when the needle rises after falling to the needle bottom dead center, and form a stitch by intertwining the needle thread and the bobbin thread with each other.
In order to form an appropriate stitch by the needle and bobbin threads, it is necessary to adjust a stitch balancing thread tension appropriately in accordance with a sewing condition. When the tension of the needle thread is excessive in the tension balancing between the needle thread and the bobbin thread, the intertwining point of the needle thread and the bobbin thread is exposed on the front surface of cloth. Conversely, when the tension of the bobbin thread is excessive, the intertwining point of the bobbin thread and the needle thread is exposed on the back surface of the cloth. No intertwining point is formed inside the cloth. In addition, a cloth shrinkage may be caused, or a formed stitch may be weak. The tension of the needle thread and that of the bobbin thread depend on, for example, the supply amount of the needle thread and that of the bobbin thread.
The supply amount of the needle thread is controlled through the reel-out of the needle thread, tension easing of the needle thread, and draw-up of the needle thread by the thread take-up lever, and an automatic stitch balancing thread tension adjusting mechanism. The supply amount of the bobbin thread is adjusted by producing tentative tension to the bobbin thread through an up-and-down action of a bobbin thread supply member that catches the bobbin thread from the lower side (see, for example, JPS62-2998 A). According to this bobbin-thread supply adjusting scheme, the pull-down amount of the bobbin-thread supply member is changed in accordance with a sewing condition, such as a pattern to be sewn, the feeding amount of cloth, a needle amplitude, the kind of cloth, and the kind of thread, thereby changing the supply amount of the bobbin thread in accordance with the sewing condition.
The bobbin-thread supply member shown in JPS62-2998 A is fixed to a shaft, and is swingable around this shaft. Arms fixed to the shaft are each swingably supported by a fork through a pin, the fork is swung around the pin by, a triangle cam which is fixed to a lower shaft and which is rotated by a sewing-machine motor through the lower shaft, a square die swingably supported by a pin near the middle location slides along the groove of an adjuster, and thus the arm swings.
That is, drive power to actuate the bobbin-thread supply member is obtained from the triangle cam fixed to the lower shaft. In this case, a timing at which the bobbin-thread supply member is actuated is limited by the phase of the triangle cam fixed to the lower shaft. Accordingly, the bobbin thread can be reeled out only at a uniform phase for various sewing conditions.
For example, in the case of zigzag stiches, it is necessary to supply a larger amount of bobbin thread than that of straight stitches until the thread take-up lever reaches the top dead center. According to the bobbin-thread supply member shown in JPS62-2998 A, however, since the bobbin thread supply phase is unadjustable, the bobbin thread is not supplied at an appropriate timing, such that the bobbin thread supply phase is advanced in the case of zigzag stitches and the bobbin thread supply phase is retarded in the case of the straight stitch, in accordance with a sewing condition.